1. Field of the Invention
The present invention relates to a particulate matter detection element for detecting particulate matter consisting primarily of soot made up of carbon contained in a measured gas emitted from an internal combustion engine of a vehicle, for example, and relates to a method of manufacturing the particulate matter detection element.
2. Description of Related Art
It is known to provide a diesel particulate filter (referred to as “DPF” hereinafter) in an exhaust gas passage of a vehicle-use diesel engine or the like to collect environmental pollutant contained in exhaust gas, particularly, soot particles and particulate matter (may be referred to as “PM” hereinafter) composed mostly of SOF (soluble organic fraction). The DPF, which is made of a porous ceramic having high resistance to heat, collects PM while the exhaust gas passes through partition walls having many pores. If a collection amount of PM exceeds a permissible value, the pressure drop of the DPF increases due to clogging, and the DPF may be broken by a heat generated at the time of burning off collected PM, causing PM to directly pass through the DPF.
There are many proposals for PM detection sensors for detecting PM contained in a measured gas, which enables determining an appropriate time to regenerate a DPF, and early and reliable detection of an abnormality such as passing of PM directly through the DPF. For example, Japanese Patent Application Laid-open No. 2005-164554 discloses a PM detection element in which PM contained in a measured gas is collected between a pair of comb-shaped electrodes disposed on the surface of an insulator so as to be opposed to each other, and the PM content of the measured gas is determined by measuring an electrical characteristic such as resistance, capacitance or impedance between the pair of the comb-shaped electrodes, which vary depending on the collection amount of PM. For another example, Japanese Patent Application Laid-open No. 2012-47596 discloses a particulate matter detection device including an element-base material of a plate-like shape, a pair of measuring electrodes disposed in the element-base material, a characteristic measuring means for measuring an electrical characteristic between the pair of the measuring electrodes, and calculating means for calculating an amount of particulate matter collected in and in the vicinity of the pair of the measuring electrodes, based on the electrical characteristic measured by the characteristic measuring means, where each of the measuring electrodes is a comb-shaped electrode including a plurality of substantially planar comb tooth portions and a comb base portion connecting the comb tooth portions together at their ends, the comb tooth portions of one of the measuring electrodes and the comb tooth portions of the other measuring electrode being located so as to alternate with each other, the comb base portion of at least one of the measuring electrodes being covered by a comb base covering portion made of dielectric. This particulate matter detection device detects PM trapped in or in the vicinity of the pair of the measuring electrodes by measuring variation of an electrical characteristic between the pair of the measuring electrodes.
Such a conventional PM detection element is formed to have a comb pattern in which a plurality of reed-shaped electrodes are disposed spaced out from one another so that different polarities alternate on the surface of an insulating substrate such as alumina or a conductive substrate such as zirconia, using a thick film printing method or a thin film printing method such as chemical vapor deposition (CVD) or physical vapor deposition (PVD). There is a dead mass in such a PM detection element including the comb-shaped electrodes opposed to each other. Before the mass of PM collected between the comb-shaped electrodes exceeds the dead mass, the PM detection element cannot sense PM. Accordingly, it is necessary to reduce the dead mass as much as possible to enable early and reliable detection of an abnormality of the PDF.
On the other hand, if the mass of PM collected between the comb-shaped electrodes exceeds a certain limit value, since the electrical characteristic between the comb-shaped electrodes becomes saturated and remains unchanged, PM contained in the measured gas cannot be measured. Accordingly, the PM detection device as described above is configured to burn off PM collected between the comb-shaped electrodes when the amount of collected PM reaches the limit value using a heater to regenerate the PDF.
However, when the comb-shaped electrodes are formed using a common thick film printing method, the distance between the adjacent comb tooth portions is about 20 μm at minimum because of rheology characteristics of a printing paste used and constraints in manufacturing masks to be formed in a printing screen. On the other hand, when the comb-shaped electrodes are formed using a thin film printing method such as CVD or PVD, although it is possible to form an extremely fine pattern, the facility cost therefor becomes very high. Accordingly, the manufacturing cost increases in this case. In addition, since the comb-shaped electrodes are inevitably a thin film, when they are used in a severe atmosphere in which they are subjected to a thermal stress occurring at the time of burning off collected PM, or a hot/cold stress due to moisture contained in the measured gas, the comb-shaped electrodes may evaporate or flake off.
Further, it was found that if the pair of the comb-shaped electrodes are formed such that the distance between the adjacent comb tooth portions is in the order of 20 to 50 μm using a common thick film printing method, the dead mass is inadmissibly large and varies greatly from device to device as explained later.